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Blood vessels begin to form from the embryonic mesoderm. The precursor hemangioblasts differentiate into angioblasts, which give rise to the blood vessels and pluripotent stem cells that differentiate into the formed elements of the blood. Together, these cells form blood islands scattered throughout the embryo. Extensions known as vascular tubes eventually connect the vascular network. As the embryo grows within the mother’s womb, the placenta develops to supply blood rich in oxygen and nutrients via the umbilical vein and to remove wastes in oxygen-depleted blood via the umbilical arteries. Three major shunts found in the fetus are the foramen ovale and ductus arteriosus, which divert blood from the pulmonary to the systemic circuit, and the ductus venosus, which carries freshly oxygenated blood high in nutrients to the fetal heart. INTERACTIVE LINK QUESTIONS 1. Watch this video (http://openstaxcollege.org/l/ capillaryfunct) to explore capillaries and how they function in the body. Capillaries are never more than 100 micrometers away. What is the main component of interstitial fluid? REVIEW QUESTIONS 3. The endothelium is found in the ________. a. tunica intima 2. Listen to this CDC podcast (http://openstaxcollege.org/l/CDCpodcast) to learn about hypertension, often described as a “silent killer.” What steps can you take to reduce your risk of a heart attack or stroke? 5. Closer to the heart, arteries would be expected to have a higher percentage of ________. b.

tunica media c. tunica externa d.

lumen c.

4. Nervi vasorum control ________. a. vasoconstriction b. vasodilation c.

capillary permeability d.both vasoconstriction and vasodilation

endothelium smooth muscle fibers elastic fibers collagenous fibers a. b. d. 972 Chapter 20 | The Cardiovascular System: Blood Vessels and Circulation 6. Which a. b. c. d. of the following best describes veins? thick walled, small lumens, low pressure, lack valves thin walled, large lumens, low pressure, have valves thin walled, small lumens, high pressure, have valves thick walled, large lumens, high pressure, lack valves 14. Net a. b. c. d.

filtration pressure is calculated by ________. adding the capillary hydrostatic pressure to the interstitial fluid hydrostatic pressure subtracting the fluid drained by the lymphatic vessels from the total fluid in the interstitial fluid adding the blood colloid osmotic pressure to the capillary hydrostatic pressure subtracting the blood colloid osmotic pressure from the capillary hydrostatic pressure 7. An especially leaky type of capillary found in the liver and certain other tissues is called a ________. a.

capillary bed b. fenestrated capillary c. sinusoid capillary d.

metarteriole

8. In a blood pressure measurement of 110/70, the number 70 is the ________. a.

systolic pressure b. diastolic pressure c. pulse pressure d.

mean arterial pressure

9. A healthy elastic artery ________. a. is compliant b.

reduces blood flow c.

is a resistance artery d. has a thin wall and irregular lumen

10. Which of the following statements is true? a. The longer the vessel, the lower the resistance and the greater the flow. b.

As blood volume decreases, blood pressure and

blood flow also decrease. c. true.

Increased viscosity increases blood flow. d.

All of the above are

11. Slight vasodilation in an arteriole prompts a ________. a.

slight increase in resistance b. huge decrease in resistance

huge increase in resistance c.

12. Venoconstriction increases which of the following? a. b.

blood flow within the vein c.

slight decrease in resistance d.

blood pressure within the vein

return of blood to the heart d. all of the above

13. Hydrostatic pressure is ________. a. greater than colloid osmotic pressure at the venous end of the capillary bed b. the pressure exerted by fluid in an enclosed space c. zero at the midpoint of a capillary bed d. all of the above 15. Which of the following statements is true? a. filtration than enters through reabsorption. b.

about

In one day, more fluid exits the capillary through In one day, approximately 35 mm of blood are

filtered and 7 mm are reabsorbed. c. In one day, the capillaries of the lymphatic system absorb about 20.4 liters of fluid. d.

None of the above are true.

16. Clusters of neurons in the medulla oblongata that regulate blood pressure are known collectively as ________. a. baroreceptors b. angioreceptors c. 17.

In ________.

the cardiomotor mechanism d. the cardiovascular center

This OpenStax book is available for free at http://cnx.org/content/col11496/1.8 the renin-angiotensin-aldosterone mechanism, a.

decreased blood pressure prompts the release of renin from the liver

b.

aldosterone prompts increased urine output c. aldosterone prompts the kidneys to reabsorb

sodium d.

all of the above

18. In the myogenic response, ________. a.

muscle contraction promotes venous return to the

heart b.ventricular contraction strength is decreased c. vascular smooth muscle responds to stretch d. endothelins dilate muscular arteries 19. A form of circulatory shock common in young children with severe diarrhea or vomiting is ________. a. b. c. d. 20. The a. b. c. d. hypovolemic shock anaphylactic shock obstructive shock hemorrhagic shock coronary arteries branch off of the ________. aortic valve ascending aorta aortic arch thoracic aorta Chapter 20 | The Cardiovascular System: Blood Vessels and Circulation 973 21. Which of the following statements is true? a.

The left and right common carotid arteries both

branch off of the brachiocephalic trunk. b. The brachial artery is the distal branch of the axillary artery. c. The radial and ulnar arteries join to form the palmar arch. d. All of the above are true. 22. Arteries serving the stomach, pancreas, and liver all branch from the ________. a.

superior mesenteric artery b.

inferior mesenteric artery c.

celiac trunk d. splenic artery

23. The right and left brachiocephalic veins ________. 25. Blood islands are ________. a. pluripotent stem cells scattered

clusters of blood-filtering cells in the placenta b. masses of

throughout the fetal bone marrow c. vascular tubes that give rise to the embryonic tubular heart d. masses of developing blood vessels and formed elements scattered throughout the embryonic disc

26. Which of the following statements is true? a.

Two umbilical veins carry oxygen-depleted

blood from the fetal circulation to the placenta. b.

One umbilical vein carries oxygen-rich blood

from the placenta to the fetal heart. c. Two umbilical arteries carry oxygen-depleted blood to the fetal lungs. d.

None of the above are true.

27. The ductus venosus is a shunt that allows ________. a. fetal blood to flow from the right atrium to the left atrium b.

fetal blood to flow from the right ventricle to the left ventricle

c. most freshly oxygenated blood to flow into the fetal heart d.

most oxygen-depleted fetal blood to flow directly into the fetal pulmonary trunk

a. b. c. d. drain blood from the right and left internal jugular veins drain blood from the right and left subclavian veins drain into the superior vena cava all of the above are true 24. The digestive organs to the ________. hepatic portal system delivers blood from the a. liver b. hypothalamus c. spleen d.

left atrium

974 Chapter 20 | The Cardiovascular System: Blood Vessels and Circulation CRITICAL THINKING QUESTIONS 28. Arterioles are often referred to as resistance vessels. Why? 29. Cocaine use causes vasoconstriction. Is this likely to increase or decrease blood pressure, and why? 30. A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. What type of vessel is this? 31. You measure a patient’s blood pressure at 130/85. Calculate the patient’s pulse pressure and mean arterial pressure. Determine whether each pressure is low, normal, or high. 32. An obese patient comes to the clinic complaining of swollen feet and ankles, fatigue, shortness of breath, and often feeling “spaced out.” She is a cashier in a grocery store, a job that requires her to stand all day. Outside of work, she engages in no physical activity. She confesses that, because of her weight, she finds even walking uncomfortable. Explain how the skeletal muscle pump might play a role in this patient’s signs and symptoms.

33. A patient arrives at the emergency department with dangerously low blood pressure. The patient’s blood colloid osmotic pressure is normal. How would you expect this situation to affect the patient’s net filtration pressure? 34. True or false? The plasma proteins suspended in blood cross the capillary cell membrane and enter the tissue fluid via facilitated diffusion. Explain your thinking. 35. A patient arrives in the emergency department with a blood pressure of 70/45 confused and complaining of thirst. Why? 36. Nitric oxide is broken down very quickly after its release. Why? 37. Identify the ventricle of the heart that pumps oxygen- depleted blood and the arteries of the body that carry oxygen-depleted blood. 38. What organs do the gonadal veins drain? 39. What arteries play the leading roles in supplying blood to the brain? 40. All tissues, including malignant tumors, need a blood supply. Explain why drugs called angiogenesis inhibitors would be used in cancer treatment. 41. Explain the location and importance of the ductus arteriosus in fetal circulation. This OpenStax book is available for free at http://cnx.org/content/col11496/1.8 Chapter 21 | The Lymphatic and Immune System

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21 | THE LYMPHATIC AND IMMUNE SYSTEM Figure 21.1 The Worldwide AIDS Epidemic (a) As of 2008, more than 15 percent of adults were infected with HIV in certain African countries. This grim picture had changed little by 2012. (b) In this scanning electron micrograph, HIV virions (green particles) are budding off the surface of a macrophage (pink structure). (credit b: C. Goldsmith) 976

Chapter 21 | The Lymphatic and Immune System

Introduction In June 1981, the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, published a report of an unusual cluster of five patients in Los Angeles, California. All five were diagnosed with a rare pneumonia caused by a fungus called Pneumocystis jirovecii (formerly known as Pneumocystis carinii). Why was this unusual? Although commonly found in the lungs of healthy individuals, this fungus is an opportunistic pathogen that causes disease in individuals with suppressed or underdeveloped immune systems. The very young, whose immune systems have yet to mature, and the elderly, whose immune systems have declined with age, are particularly susceptible. The five patients from LA, though, were between 29 and 36 years of age and should have been in the prime of their lives, immunologically speaking. What could be going on? A few days later, a cluster of eight cases was reported in New York City, also involving young patients, this time exhibiting a rare form of skin cancer known as Kaposi’s sarcoma. This cancer of the cells that line

the blood and lymphatic vessels was previously observed as a relatively innocuous disease of the elderly. The disease that doctors saw in 1981 was frighteningly more severe, with multiple, fast-growing lesions that spread to all parts of the body, including the trunk and face. Could the immune systems of these young patients have been compromised in some way? Indeed, when they were tested, they exhibited extremely low numbers of a specific type of white blood cell in their bloodstreams, indicating that they had somehow lost a major part of the immune system. Acquired immune deficiency syndrome, or AIDS, turned out to be a new disease caused by the previously unknown human immunodeficiency virus (HIV). Although nearly 100 percent fatal in those with active HIV infections in the early years, the development of anti-HIV drugs has transformed HIV infection into a chronic, manageable disease and not the certain death sentence it once was. One positive outcome resulting from the emergence of HIV disease was that the public’s attention became focused as never before on the importance of having a functional and healthy immune system. 21.1 | Anatomy of the Lymphatic and Immune Systems By the end of this section, you will be able to: Chapter Objectives After studying this chapter, you will be able to: • Identify the components and anatomy of the lymphatic system •Discuss the role of the innate immune response against pathogens • Describe the power of the adaptive immune response to cure disease • Explain immunological deficiencies and over-reactions of the immune system • Discuss the role of the immune response in transplantation and cancer •Describe the interaction of the immune and lymphatic systems with other body systems ••• Describe the structure and function of the lymphatic tissue (lymph fluid, vessels, ducts, and organs) Describe the structure and function of the primary and secondary lymphatic organs Discuss the cells of the immune system, how they function, and their relationship with the lymphatic system The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system, for most people, is associated with the immune system to such a degree that the two systems are virtually indistinguishable. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems. Functions of the Lymphatic System A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space—that is, spaces between This OpenStax book is available for free at http://cnx.org/content/col11496/1.8 Chapter 21 | The Lymphatic and Immune System

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individual cells in the tissues. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences. As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, the transport of dietary lipids and fat-soluble vitamins absorbed in the gut uses this system. Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system. Structure of the Lymphatic System The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body (Figure 21.2). Visit this website (http://openstaxcollege.org/l/lymphsystem) for an overview of the lymphatic system. What are the three main components of the lymphatic system? 978

Chapter 21 | The Lymphatic and Immune System

Figure 21.2 Anatomy of the Lymphatic System the larger lymphatic vessels in the torso.

Lymphatic vessels in the arms and legs convey lymph to

A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck. Lymphatic Capillaries Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body (Figure 21.3). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.

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Figure 21.3 Lymphatic Capillaries Lymphatic capillaries are interlaced with the arterioles and venules of the cardiovascular system. Collagen fibers anchor a lymphatic capillary in the tissue (inset). Interstitial fluid slips through spaces between the overlapping endothelial cells that compose the lymphatic capillary. Lymphatic capillaries are formed by a one cell-thick layer of endothelial cells and represent the open end of the system, allowing interstitial fluid to flow into them via overlapping cells (see Figure 21.3). When interstitial pressure is low, the endothelial flaps close to prevent “backflow.” As interstitial pressure increases, the spaces between the cells open up, allowing the fluid to enter. Entry of fluid into lymphatic capillaries is also enabled by the collagen filaments that anchor the capillaries to surrounding structures. As interstitial pressure increases, the filaments pull on the endothelial cell flaps, opening up them even further to allow easy entry of fluid. In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In the small intestine, dietary triglycerides combine with other lipids and proteins, and enter the lacteals to form a milky fluid called chyle. The chyle then travels through the lymphatic system, eventually entering the bloodstream. Larger Lymphatic Vessels, Trunks, and Ducts The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 21.3). The superficial and ...